1. Field of the Invention
The present invention relates to a surface acoustic wave filter and a communication apparatus using this.
2. Related Art of the Invention
In recent years, progress in information communication field has given rise to increase in information amount to be transmitted. Accompanied thereby, a surface acoustic wave filter that is excellent in phase linearity for a comparatively wide band range and within a passband, and conventionally, a surface acoustic wave filter of transversal type was suitable as such a surface acoustic wave filter. On the other hand, as well known, since a surface acoustic wave filter of transversal type gives rise to a large insertion loss, as technology to enhance low insertion loss, a surface acoustic wave filter of transversal type with a unidirectional electrode being utilized is being expected and considered.
A conventional surface acoustic wave filter of transversal type with a unidirectional electrode being utilized will be described below.
FIG. 10(a) is a drawing showing a conventional surface acoustic wave filter of transversal type with a unidirectional electrode being utilized. Reference numeral 1001 denotes a piezoelectric substrate, and a surface acoustic wave filter is constructed by forming an input and output interdigital transducer electrode (referred to as IDT electrode hereinafter) on the piezoelectric substrate 1001 and disposing the input IDT electrode 1002 and the output IDT electrode 1003 at a predetermined distance. The input IDT electrode 1002 as well as the output IDT electrode 1003 includes a first as well as a second unidirectional electrodes 1004 and 1005 according to a prior art disclosed in the IEEE Ultrasonics symposium, 1989, PP. 77-89. Incidentally the entire disclosure of above document are incorporated herein by reference in its entirely.
FIG. 10(b) shows an enlarged view of the first unidirectional electrode 1004. As a basic construction, a basic unit is constructed by one xcex/4-width electrode finger 1004a with a quarter width of a wavelength xcex of a surface acoustic wave propagating on the piezoelectric substrate 1001 and two xcex/8-width electrode fingers 1004b, totaling three electrode fingers. In addition, the second unidirectional electrode 1005 and the unidirectional electrode 1004 are symmetrical. The unidirectional electrode of such type is called EWC-SPUDT (Electrode Width Controlled Single-Phase Unidirectional Transducer), and has been conventionally used as technology of lowering insertion loss.
For the surface acoustic wave filter constructed as described above, frequency characteristics of a filter are determined by thinning out electrode fingers of the input IDT electrode 1002 and the output IDT electrode 1003 and giving various kinds of weighing. In the prior arts, thus, flat filter characteristics within the passband for a wide band range and attenuation amount that is steep in the vicinity of the passband are realized.
In addition, in Japanese Patent Laid-Open No. 2000-77974 specification, a surface acoustic wave filter having a first and second channel on a piezoelectric substrate is disclosed. In FIG. 11, a schematic construction view of a surface acoustic wave filter having a first and a second channel according to a prior art is shown. The above described two-channel filter has two channels of a first channel 1102 and a second channel 1103 each having an input IDT electrode 1102a, 1103a and an output IDT electrode 1102b, 1103b provided on a piezoelectric substrate 1101. Such a surface acoustic wave filter is constructed to have phase characteristics on the first channel 1102 and the second channel 1103 of being in phase in the passband and in opposite phase each other the stopband. Incidentally the entire disclosure of above document are incorporated herein by reference in its entirely.
In recent years, enhancement in compactness and lightness on a portable terminal is progressed and accompanied thereby, enhancement in compactness of a surface acoustic wave filter is being demanded. However, if a steep attenuation amount in the vicinity of a passband is tried to be given in the above described surface acoustic wave filter of transversal type as shown in FIG. 10, a sufficient weighting onto the input/output IDT electrode will become necessary to make the length IDT electrode longer, giving rise to a problem that enhancement in compactness is difficult.
In addition, the surface acoustic wave of two-channel filter construction as shown in FIG. 11 has sharp band range characteristics and enables a compact surface acoustic wave filter to be provided, presenting, however, a problem that it lacks flatness within a passband.
Accordingly, the present invention has an object to provide a surface acoustic wave filter being compact, having wide passband and having steep attenuation characteristics in the vicinity of passband as well as a flat characteristics within the passband.
To achieve the object mentioned above, one aspect of the present invention is a surface acoustic wave filter, comprising:
a piezoelectric substrate; and
at least two filter tracks having at least an input interdigital transducer electrode and an output interdigital transducer electrode provided on said piezoelectric substrate,
wherein the respective input interdigital transducer electrodes of said at least two filter tracks are brought into connection in parallel;
the respective output interdigital transducer electrodes of said at least two filter tracks are brought into connection in parallel, and
overlapping widths of electrode fingers of said input interdigital transducer electrode and output interdigital transducer electrode are different from each other for each of said at least two filter tracks.
Another aspect of the present invention is the surface acoustic wave filter,
wherein said filter tracks are two filter tracks of a first filter track and a second filter track,
the input interdigital transducer electrodes of said first and second filter tracks are brought into connection in parallel,
the output interdigital transducer electrodes of said first and second filter tracks are brought into connection in parallel, and
overlapping width of the electrode fingers of the said input interdigital transducer electrode and output interdigital transducer electrode in said first filter track and overlapping width of the electrode fingers of the said input interdigital transducer electrode and output interdigital transducer electrode in said second filter track are different from each other.
Still another aspect of the present invention is the surface acoustic wave filter, wherein said first filter track and said second filter track have different weighting functions.
Yet still another aspect of the present invention is the surface acoustic wave filter, wherein a phase relationship between said first filter track and said second filter track is substantially in-phase within a passband, and is substantially in opposite phase outside the passband, and center frequencies of said first and second filter tracks are substantially the same.
Still yet another aspect of the present invention is the surface acoustic wave filter, wherein said first filter track has a transmission characteristic of having two peaks within the passband, said second filter track has a transmission characteristic of having one peak within the passband; and an amplitude characteristic of said first filter track and said second filter track is that frequencies of values lower by substantially 3 dB than a value of a maximum attenuation amount are substantially the same.
A further aspect of the present invention is the surface acoustic wave filter, wherein with the overlapping width of said first filter track being W1 and the overlapping width of said second filter track being W2, a relationship of 0.8xe2x89xa6W1/W2xe2x89xa60.95 is fulfilled.
A still further aspect of the present invention is the surface acoustic wave filter, wherein at least one of said input interdigital transducer electrode and/or output interdigital transducer electrode includes a unidirectional electrode.
A yet further aspect of the present invention is the surface acoustic wave filter, wherein said unidirectional electrode includes two kinds, that is, a first unidirectional electrode to intensify surface acoustic waves in a predetermined direction and a second unidirectional electrode to intensify surface acoustic waves in the direction opposite to said predetermined direction.
A still yet further aspect of the present invention is the surface acoustic wave filter, wherein parallel connection of said input interdigital transducer electrodes of said filter track is configured by connecting adjacent electrode fingers with each other, and
parallel connection of said output interdigital transducer electrodes of said filter track is configured by connecting adjacent electrode fingers with each other.
An additional aspect of the present invention is a surface acoustic wave filter, comprising:
a piezoelectric substrate; and
at least one filter track having an input interdigital transducer electrode and an output interdigital transducer electrode provided on said piezoelectric substrate,
wherein said input interdigital transducer electrode and/or said output interdigital transducer electrode include a first unidirectional electrode to intensify surface acoustic waves in one direction and a second unidirectional electrode to intensify surface acoustic waves in the direction opposite to said one direction.
A still additional aspect of the present invention is the surface acoustic wave filter, wherein said first and second unidirectional electrodes have four electrode fingers in one wavelength,
said four electrode fingers have two electrode finger pairs, and
said electrode finger pairs have electrode fingers having different widths, and an electrode width ratio (L2/L1) between a width (L2) of thick electrode finger and width (L1) of thin electrode finger is larger than 1.
A yet additional aspect of the present invention is the surface acoustic wave filter, wherein, in said electrode finger pair,
with a distance between said thin electrode finger and said thick electrode finger being xcex3;
with a distance between said thick electrode finger and the left end of a region covering said input interdigital transducer electrode and/or said output interdigital transducer electrode sectioned by a half-wavelength unit being xcex1; and
with a distance between said thin electrode finger and the right end of a region covering said input interdigital transducer electrode and/or said output interdigital transducer electrode sectioned by a half-wavelength unit being xcex2, relationships of xcex3 greater than xcex1+xcex2 and xcex1 less than xcex2 are given.
A still yet additional aspect of the present invention is the surface acoustic wave filter, wherein said electrode width ratio (L2/L1) in said electrode finger pair falls within a range of 1.4xe2x89xa6(L2/L1)xe2x89xa63.6.
A supplementary aspect of the present invention is the surface acoustic wave filter, wherein positions of said thin electrode finger and said thick electrode finger of said first unidirectional electrode are opposite from positions of said thin electrode finger and said thick electrode finger of said second unidirectional electrode.
A still supplementary aspect of the present invention is the surface acoustic wave filter, wherein said piezoelectric substrate is 28xc2x0 to 42xc2x0 rotating Y cut quartz substrate.
A yet supplementary aspect of the present invention is a communication apparatus, comprising:
the surface acoustic wave filter;
transmission means of carrying out transmission; and reception means of carrying out reception.